Spread spectrum is a communications technique where the baseband signal bandwidth is intentionally spread over a larger bandwidth by modulating the signal with a higher-frequency “spreading” code prior to transmission. As a result, energy used in transmitting the baseband signal is spread over a wider bandwidth, and appears noiselike. The ratio between the spread baseband and the original signal is called processing gain. Typical Spread spectrum processing gains vary between 10 dB and 60 dB. A complementary de-spreading operation is performed at the receiver. Example spread spectrum techniques include frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS), each of which provide a reliable communications method for users.
In the presence of many users, however, co-channel interference degrades the performance of existing spread spectrum techniques. Media access control (MAC) must be implemented to reduce co-channel interference and to coordinate access to the channel by all interested, cooperating parties. In addition, it is desirable in some applications that the transmitted waveform has a low probability of detection (LPD) by unintended receivers, and that the transmitted waveform is resilient in the presence of jamming signals (i.e., AJ).
None of the known existing spread-spectrum techniques provide a signal that has suitable LPD and AJ performance, while simultaneously allowing for conferencing without a MAC, and easy implementation. In addition, each known technique is associated with one or more disadvantages, such as wasteful use of bandwidth in DSSS, performance degradation in the presence of burst errors, and performance degradation in the presence of interference, both hostile and non-hostile.
What is needed, therefore, is an improved differential M-ary coding scheme having one or more hopped features, where the coding scheme can optionally be combined with an iterative or non-iterative form of multiuser detection (MUD).